The invention relates generally to an apparatus and method for controlling the operation of a centrifugal chiller refrigeration system. More particularly, the chiller control system of the present invention operates a centrifugal chiller which possesses both a magnetic bearing centrifugal compressor and an adjustable speed motor drive. Additionally, the present invention discloses the necessary components and control logic for use with the chiller control system.
A centrifugal chiller typically consists of the following components: one or more evaporators, compressors, condensers and expansion devices. In a chiller, the compressor acts as a vapor pump, where raising the pressure of the refrigerant from the evaporating pressure to the condensing pressure provides an active means of absorbing heat from a lower temperature environment and rejecting that heat to a higher temperature environment. As an active machine, the chiller requires an apparatus to control its operation.
In general terms, a centrifugal compressor for a chiller typically consists of the following components: inlet guide vanes, one or more impellers within a housing surrounded by one or more diffusers with collectors driven by some mechanical shaft means, such as for example, an electric motor. The mechanical shaft means is supported by one or more bearings of the rolling element, journal, or magnetic bearing type which accommodate both radial and axial loads. In variable speed electric chillers, the centrifugal compressor is supplied with electrical power through an adjustable speed motor drive which alters the frequency and/or voltage of the power to the motor to modulate the speed of the compressor.
The chiller control system for a centrifugal chiller typically performs one or more of the following functions: adjust inlet guide vane position and/or compressor speed to match the cooling capacity with the cooling load, monitor chiller operating conditions for unsafe operation and take appropriate action when encountered, display chiller operating conditions for user interpretation, and/or operate the chiller in response to a predefined schedule.
Chiller control systems of the microprocessor type typically consist of one or more of the following devices, a microprocessor which runs a control algorithm, sensors which acquire operating data from one or more points on the chiller, display devices for communicating information on chiller operating conditions and various devices for the input of information to the chiller control system. While these chiller control systems have performed adequately for centrifugal chillers consisting of compressors with rolling element bearings and/or journal bearings, they are inadequate for chillers with magnetic bearing centrifugal compressors.
Rolling element bearings are generally passive devices and, during normal operation, operate without the requirement of active control. The chiller control system does not typically provide active control of the rolling element bearings where, in this context, active implies continual adjustment of some bearing feature. Chiller control systems for centrifugal chillers which use rolling element bearings in the compressor may monitor the bearing temperature, at periodic intervals, as an indication of whether the machine is operating properly. An elevated temperature is used as an indication of a potential mechanical problem with the bearings. If the measured bearing temperature exceeds a predefined setpoint, the chiller control system may be programmed to stop the machine and alert the user.
In magnetic bearing centrifugal compressors, the compressor rotor is suspended on a magnetic field generated in the magnetic bearings. For definitional purposes, xe2x80x9cmagnetic bearingsxe2x80x9d are electromagnetic devices used for suspending a rotating body in a magnetic field without mechanical contact. The bearings can be further classified as active, indicating that some type of active control system is necessary to ensure stable levitation of the rotating body.
Distinct from other compressor types, a magnetic bearing centrifugal compressor uses magnetic bearings as the primary means for supporting the rotor structure. There is a clearance gap between the rotating and stationary components of the bearing that is measurable and controllable. For the magnetic bearings to operate properly, electrical power and proper operation of the magnetic bearing control electronics are required.
As described previously, existing chiller control systems for centrifugal chillers do not work adequately for centrifugal chillers with active magnetic bearing centrifugal compressors. The necessary control strategies are not provided by the controllers known in the art.
Specifically, these chiller control systems do not monitor the magnetic bearings for stable levitation which is required in order to prevent damage to the magnetic bearing centrifugal compressor. Existing chiller control systems may allow the compressor to turn at high speeds while the magnetic bearings are not stably levitated. When this occurs, the rotor does not spin about a fixed axis. Rather, the rotor spins on an axis contained within a small cylinder defined by the clearances between the compressor rotor assembly and the stationary compressor housing. The unconfined rotation of the compressor rotor assembly may generate large forces (due to the kinetic energy stored in the rotor at high speeds), and may thereby damage the magnetic bearings, the compressor rotor assembly, and compressor impeller, as well as the attached stationary compressor housing. In the event of a loss of active control of the compressor rotor, the rotor may contact the auxiliary bearings within the compressor.
Due to the disadvantages associated with chiller control systems known in the prior art for centrifugal chillers which have magnetic bearing centrifugal compressors, it should therefore be appreciated that there is a need for a chiller control system for a magnetic bearing centrifugal chiller.
In view of the foregoing, it is an object of the present invention to provide a chiller control system apparatus and method for controlling a centrifugal chiller which possesses a magnetic bearing centrifugal compressor and an adjustable frequency motor drive.
The function of a chiller control system is to operate a centrifugal chiller in such a manner as to meet the cooling load requirements. The chiller control system continuously monitors the cooling load and other chiller variables, and adjusts the operation of the chiller to match the cooling load. In sophisticated chiller control systems, in addition to matching cooling load, the control system seeks to operate the compressor in a manner that maximizes operating efficiency to reduce overall electrical power consumption.
While maximizing overall centrifugal chiller operating efficiency, the chiller control system must operate the magnetic bearing centrifugal compressor safely by avoiding compressor surge. Surge occurs when there are sudden reversals in the direction of fluid flow through the compressor impeller as the pressure difference across the impeller becomes too large. (Since additional static pressure rise occurs in the compressor diffuser as the fluid is decelerated, the pressure near the diffuser entrance may exceed the pressure at the impeller exit.)
When the impeller exit pressure drops below diffuser pressure, the fluid flow direction reverses and flows back into the compressor impeller, resulting in significantly increased stresses and a substantially increased vibration of the compressor rotor. The flow reversal causes the pressure at the impeller exit and within the diffuser to drop. When the pressure drops below the surge point, the flow again reverses direction and flows into the diffuser. A compressor operates in a surging condition when these sudden flow reversals are occurring. The flow reversals during surge damage the chiller equipment.
Prior experimental studies have shown that the maximum operating efficiency of a centrifugal compressor is close to the surge boundary. To minimize energy consumption, the impeller should not impart more energy to the fluid than necessary to meet the temperature lift requirements for the vapor compression refrigeration cycle. Any additional energy imparted to the refrigerant flow above the required amount is wasted. Maximum efficiency occurs near the surge boundary. Hence, to maximize the efficiency of a centrifugal chiller, the compressor should be operated at the lowest speed possible that is just great enough to avoid a surge condition. The location of the surge point is a function of the aerodynamic design of the centrifugal compressor.
During centrifugal compressor development, detailed measurements of the pressure rise versus flow rate behavior of the compressor at various operating speeds, inlet guide vane angle settings and diffuser vane angle settings are typically conducted. These measurements determine a surge line for the compressor, a plot of the points (flow coefficient, head coefficient) on the compressor operating map (where the non-dimensional head coefficient lies along the y-axis and the non-dimensional flow coefficient lies along the x-axis) where the surge condition is encountered. The compressor avoids a surging condition when its current operating state (defined by the calculated flow coefficient and head coefficient) lies below and to the right of the surge line on the compressor operating map. The operating envelope for the compressor is the complete set of points (flow coefficient, head coefficient) for which some combination of inlet guide vane angle, diffuser vane angle, and compressor speed will allow operation in a non-surge condition. This operating map for the compressor can be stored in the memory of the control system as a set of equations which define the surge line or as a set of points which form an array of stable operating states.
A surge condition can be detected by the chiller control system by changes in chiller performance. When the compressor is surging, the torque on the rotor oscillates (from positive to negative) which causes noticeable changes in the electrical current supplied to the motor element.
A surge condition can also be detected by the chiller control system by changes in the magnetic bearing operating conditions. When the compressor is surging, the rotor oscillates which causes noticeable changes in bearing position, stabilizing current, force and temperature.
The compressor head coefficient-flow coefficient operating map determines the safe operating condition (flow coefficient, head coefficient) for a particular cooling load and pressure lift requirement. The compressor head-flow operating map can be adjusted or modified, should changes occur over time in either the impeller surface finish, the diffuser vane condition or the impeller to shroud clearance.
The typical chiller control system adjusts the compressor speed, inlet guide vane position, and diffuser vane position to meet the pressure ratio requirements and the cooling load requirements while operating as efficiently as possible. Prior experimental research studies have shown that a coordinated adjustment of the inlet guide vanes and diffuser vanes can increase the operating efficiency of a centrifugal compressor impeller from 2 to 6 percent. Wallman et al., xe2x80x9cImprovements in Performance Characteristics of Single-Stage and Multistage Centrifugal Compressors by Simultaneous Adjustments of Inlet Guide Vanes and Diffuser Vanes.xe2x80x9d Transactions of the ASME Journal of Turbomachinery, January 1987, Vol. 109, pgs. 41-47.
It is an object of the present invention to provide a chiller control system for centrifugal chillers which possess magnetic bearing centrifugal compressors.
It is another object of the present invention to provide a chiller control system for centrifugal chillers which possess adjustable speed motor drives.
It is yet another object of the present invention to prevent operation of the chiller in the event of a problem with the magnetic bearings, thereby preventing damage to the chiller compressor(s).
It is another object of the present invention to prevent operation of the magnetic bearings in the event of a problem with the centrifugal chiller electrical power supply, thereby prolonging magnetic bearing operating life.
Another object of the present invention is to provide a measurement of the electrical power consumption of the centrifugal chiller during operation, thereby eliminating the need for an external electrical power measurement device.
It is even another object of the present invention to provide a measurement of the centrifugal compressor operating speed.
It is yet another object of the present invention to provide a method of storing centrifugal chiller operating data over long periods of time to allow the assembly of energy usage studies.
It is still a further object of the present invention to provide an improved user interface for displaying operational parameters of the centrifugal chiller.
Yet another object of the present invention is to provide a chiller control system algorithm which controls the operation of inlet guide vane position, diffuser vane position, magnetic bearing position, and motor speed in order to maximize the chiller operating efficiency.
It is another object of the present invention to provide a method for measuring bearing forces, vibrations and imbalances in order to indicate the machine""s condition, and predict problems and schedule maintenance.
These and other objectives and advantages are achieved by the chiller control system apparatus and method according to the invention. A centrifugal chiller, for which the preferred embodiment of the invention is applicable, consists of an evaporator, a magnetic bearing centrifugal compressor, a condenser, and an expansion device. The magnetic bearing centrifugal compressor increases the pressure of the refrigerant vapor from the saturation pressure of the refrigerant in the evaporator to the saturation pressure of the refrigerant in the condenser. A typical embodiment of the magnetic bearing centrifugal compressor, such as that described in co-pending patent application Ser. No. 08/908,035, filed Aug. 11, 1997, the specification of which is herein expressly incorporated by reference, contains a compressor rotor supported on both sides of the electric motor element by radial magnetic bearings of the type well known to those skilled in the art. Axial magnetic bearings located outside of each radial magnetic bearing absorb thrust loads. A microprocessor magnetic bearing control unit (MBU) provides active control of the magnetic bearings to maintain the compressor rotor in a stable levitated position at all operating speeds. The magnetic bearing centrifugal compressor is driven by an electric motor whose speed is controlled by a microprocessor adjustable speed motor drive (ASD).
In a preferred embodiment, the chiller control system apparatus consists of a microprocessor chiller controller (CC), an adjustable speed motor drive (ASD), and a magnetic bearing control unit (MB). The chiller controller (CC) acquires, processes, records and analyzes operating data from the centrifugal chiller sensors. The chiller controller (CC) possesses both analog and digital input and output capabilities for data acquisition and control. Additionally, the chiller controller (CC) uses a touchscreen display for data input and output communication. The chiller controller (CC) runs a chiller control system algorithm (described later) that processes input sensor data and sends control signals to various other components of the chiller control system described herein.
The chiller controller (CC) communicates with the magnetic bearing control unit (MBU) through digital input and output signal lines and serial communications links. Through these lines, the CC provides commands to levitate and delevitate the magnetic bearings, monitors the operating status of the magnetic bearings, reads any alarm or warning conditions and accesses diagnostic and tuning functions. The chiller controller (CC) communicates with the adjustable speed motor drive (ASD) through both digital and analog input and output signals lines. Through these lines, the CC provides commands to stop and start the centrifugal chiller, signals the desired motor speed, monitors operating data, reads any alarm and/or warning conditions and accesses other control functions. It is through the analog input signal line of the ASD that the CC communicates the desired compressor operating speed to the ASD. The ASD then uses its internal microprocessor and PID algorithm to match actual compressor speed to the desired setpoint speed.
As critical components of the chiller control system, the MBU and the ASD are connected by pairs of incoming and outgoing signal lines. Through these lines, alarm and/or warning conditions are communicated instantly whenever they occur to the other component, thus allowing the microprocessor of the other component to take the appropriate action.
The CC actuates the inlet guide vanes through inlet guide vane position and feedback signals. The CC actuates the diffuser vanes through diffuser vane position and feedback signals. The position of the inlet guide vanes, the diffuser vanes, and the compressor operating speed are coordinated by a complex chiller control system algorithm that responds to input data from a variety of sensor signals which monitor operating conditions within the chiller. The chiller control system algorithm provides all monitoring, controlling and communicating functions.
The chiller control system algorithm according to the invention serves to operate the centrifugal compressor at the lowest speed possible with the inlet guide vanes and the diffuser vanes adjusted at an angle to maximize efficiency. Here, the chiller control system algorithm contains several loops that adjust the three main control parameters (inlet guide vane position, diffuser vane position, and compressor speed).